man i had endless powdery mildew problems in my basement that nothing helped. its really old and wet etc so it was like the environment was just saturated with weird spores etc
i used a product called microban and an ultrasonic fogger to fog the whole basement and have not had any pm for over a year.
im pretty sure microban kills viruses as well. its supposed to be basically non toxic (wear equipment when fogging obv). they use it in hospitals for that reason
if you can get your hands on a fogger you could bomb your house top to bottom (with protective gear!) close it up for a day and go back. i doubt any airborne shit will be living in there and all surfaces should be clean too. the fogger is key because it gets the microban EVERYWHERE lol
anyways just a thought, it might be something to look into